1.) Field of the Invention
This invention relates in general to a purging composition. In one aspect, this invention is directed to a composition useful as a purgant for thermoplastic processing equipment. In another aspect, this invention is directed to a process for removing contaminants from thermoplastic processing equipment by using the purging composition.
2.) Background of the Related Art
In polymer processing, the interface between the surface of the polymer melt and the metal is a very important factor affecting the surface quality of the processed product and the efficiency of processing. Since many surface active additives are used in commercial resins, the interface is easily contaminated.
Tenacious additives such as fluoroelastomers can adsorb on the metal surface itself, and if they remain on the hot metal surface under open air conditions they can degrade and become even more difficult to clean. Particles of degraded material may slough off and contaminate the processed product. Additives remaining in the adsorbed state on the metal surface can also interfere with the performance of other functional additives. Thus, it is important to have an effective purging composition to clean the processing equipment before use in a new application.
There are several commercial purging compounds currently available for use in cleaning thermoplastic processing equipment. However, none of them is very effective, especially when it comes to cleaning tenacious adsorbates like fluoroelastomers. The conventional purging compounds use mechanical approaches by employing either a low melt index (ASTM D-1238) resin or a high loading of filler particles. They may contain a chemical lubricant to ease processing. However, the chemical additives generally have not been designed to enhance competitive adsorption.
In recent years various methods have been employed for purging plastic molding machines. Purging compositions usually contain a lubricant in a thermoplastic resin. Such compositions are useful for purging thermoplastic resins such as, polyolefin resin, vinyl chloride resin, polystyrene resin, peroxide containing resins and various copolymers, polyamides and the like. By using a purging resin which contains a lubricant which is stable, the purging can be effected at a high temperature with a minimum of bubbling, fuming, smelling or degradation of resin properties.
A detergent composition is disclosed in Japanese patent publication No. 54647/1988, and consists of thermoplastic resin, 30-70 weight percent inorganic filler and 1-30 weight percent lubricant. The composition is prepared by melting and kneading the mixture and then forming into pellets. As suitable lubricants the reference mentions liquid paraffin, natural paraffin, microwax, synthetic paraffin, polyethylene wax, higher fatty acid, oxyfatty acid, higher fatty acid amide and the like. The composition is indicated to be useful for removing colored resin remaining in the forming machine as well as burnt or carbonized resin.
Thus, while several purging compositions were available prior to the present invention, none of the commercialy available purging compounds was effective, particularly when it came to cleaning tenacious adsorbates, such as fluoroelastomers. This results in extended down time while the equipment is thoroughly cleaned as well as the possibility of contamination when the processing equipment is used for new applications. Of particular importance, is the cleanliness of processing equipment utilized in pilot plant and research studies of resin systems. In such instances where the effect of small amounts of additives is studied, it is imperative that the processing equipment be free of any possible source of contamination.
Accordingly, one or more of the following objects will be achieved by the practice of the present invention. It is an object of this invention to provide a purging composition useful as a purgant for processing equipment. A further object is to provide a purgant which is particularly useful in removing contaminants from thermoplastic processing equipment. Another object is to provide a purgant composition which is ideally suited for cleaning the die of an extruder used in thermoplastic processing operations. Another object of this invention is to provide a composition comprising an adsorbate, a resin matrix, and optionally an abrasive and/or a carrier resin. A further object is to provide a purging composition which is present in a carrier resin. A still further object is to provide a purging composition which is present in a carrier resin having a melting point different from that of the matrix. A further object of the present invention is to provide a process for the preparation of the purging composition. Another object is to provide a method for cleaning thermoplastic processing equipment using the purging composition of the invention. These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.